Success in treating HIV/AIDS is critically dependent upon a thorough and accurate understanding of occult infection---where and how HIV persistence is maintained. This proposal focuses on the macrophage and follicular dendritic cell (FDC) as cellular reservoirs for HIV, and bone marrow and brain as tissue reservoirs. Although they have received comparatively little attention, these cell and tissue compartments most probably represent HIV reservoirs of considerable significance. Importantly, each appears capable of serving as a sanctuary for latent HIV infection. We propose here to determine if, indeed, these cells and tissues can serve as latently-infected hosts able to yield infectious particles that can seed the body. We seek to identify patterns of HIV trafficking between lymphoid tissues and the Central Nervous System, and determine if the brain can reseed the periphery with HIV. We also seek to identify cell and tissue reservoirs for antiretroviral drug-sensitive, versus drug-resistant, HIV. For these studies, HIV genotyping will be performed using contemporaneously-collected ante-mortem bone marrow and excisional cervical lymph node biopsy specimens, in conjunction with patient-matched plasma, cerebrospinal fluid (CSF), and separated blood monocyte and T-lymphocyte specimens collected concomitantly, as well as prior to, and following biopsy. Two regions of the HIV genome will be examined, the V1-V4 region of the envelope gene, and a 1.2kb fragment which includes the protease gene and that portion of the reverse transcriptase gene which codes for most of the antiretroviral resistance mutations of significance. To optimally address the question of whether the brain can seed the periphery with HIV, we will evaluate the HIV species present not only within the lymph node (LN) CD4+ T-cell population, but also those associated with LN germinal center follicular dendritic cells (FDC). These investigations will be complemented with studies of post-mortem tissues. In addition, we seek to identify events in bone marrow associated with systemic virologic failure. Attention will be paid to HIV expression and monocyte/macrophage activation in marrow. Immunohistochemistry and flow cytometry methods will be used. These studies will yield important new information regarding HIV reservoirs, as well as the contributions of events in bone marrow to HIV immunopathogenesis and disease progression.